Diaphragm type hydraulic press



July 9, 1968 R. A. MYERS DIAPHRAGM TYPE HYDRAULIC PRESS 2 Sheets-Sheet 1Filed Nov. 10, 1965 INVENTOR.

@aflZfl/l M4525 United States Patent Offi Patented July 9, 19683,391,559 DIAPHRAGM TYPE HYDRAULIC PRESS Richard A. Myers, RollingHills, Califi, assignor, by mesne assignments, to MacDonnell DouglasCorporation, Santa Monica, Calif.

Filed Nov. 10, 1965, Ser. No. 507,178 16 Claims. (Cl. 72-63) ABSTRACT OFTHE DISCLOSURE Sheet metal forming press with an enclosed chamber tohold form block and sheet of metal. Rubber or other yieldable diaphragmor wall is pressurally urged toward workpiece to form it about the formblock. Chamber is defined by head platen, base platen, and tension ringmeans which constitute the side walls of the chamber. In form shown,clamp rings grip margins of platens to prevent vertical separationduring forming operation. Tension ring means may be one or more rings ofmetal which are free to expand or contract radially independently of theplatens in response to variations in the forming pressure. The tensionrings absorb the radial pressure load and the platens absorb thevertical pressure load separately to prevent concentrated bendingstrains in inside corners of integral cavity.

This invention lies in the field of hydraulic diaphragm type presses inwhich the main body members of such a press define, during a formingoperation, a completely enclosed cavity or chamber containing a formblock and a workpiece of sheet metal to be formed thereon together withan expansible diaphragm adapted to apply hydraulic forming pressure tothe workpiece. The invention is directed particularly to a constructionand arrangement of such a press in which the hoop tension stresses inthe surrounding wall are isolated or segregated from the vertical loadstresses applied to the body portions defining the upper and lower wallsof the cavity so that corner bending stress concentrations are avoided.

Various types of hydraulic sheet metal forming presses are now in usewhich utilize the generally similar principle of confining a sheet metalworkpiece on a form 'block within a cavity or chamber of the press bodyhaving generally horizontally extending upper and lower walls and asurrounding side wall which is integral with one of the other walls,usually the upper one. This provides an upper body member with adownwardly opening cavity and a generally flat lower body member,separable from each other for loading and unloading. The work issupported by the bottom wall. A hydraulically expansible container,usually comprising a metal plate and an elastomeric diaphragm joined attheir edges, is mounted adjacen-t the upper wall and extends across thewidth of the cavity. An elastomeric pad is mounted directly beneath thecontainer or bag and also extends across the width of the cavity. Ahydraulic pump is used to force oil or other liquid into the containerto expand it and force the pad down toward the lower wall until itcontacts the workpiece and forces it to conform to the surface of theform block.

When the bag is fully expanded, there is substantially uniform hydraulicpressure exerted against every portion of the interior of the chamber.In most cases the working pressure is 5000 psi, and in some presses itis as high as 10,000 psi. Thus it will be seen that the vertical andradial loads are extremely high. The vertical separation loads are takenby various means such as tension bolts, clamps, and the like. The radialloads are taken by the side wall which expands and develops hooptension. This expansion is resisted by the integral connection with theupper body member and corner bending loads are developed which result invery high stress concentrationsat the inner corners of the junctionbetween the upper member and the side wall. Since the load is appliedand withdrawn during each forming operation there is a continuouslyrepeated stress reversal which produces fatique failure at the junction.While cracks may be repaired by welding, this is a very unsatisfactorysolution, so the press part is normally scrapped and replaced. This isvery expensive and time consuming, and the temporary loss of use of thepress adds further expense.

The present invention completely overcomes this major and very expensivedisadvantage of presses of conventional design. Generally speaking, itconsists in providing a construction in which the upper and lowermembers absorb only the vertical loads, which may be converted into beambending stresses, and the side wall is made independent of the upper andlower members so that it can expand independently under radial loads andconvert them into hoop tension without producing any corner bendingstresses.

The press disclosed herein, by way of example, embodies the invention ina simple but highly satisfactory arrangement. In general, the presscomprises a base on which is rigidly fixed a lower platen having ahorizontally extending upper face which is preferably flat. An upperplaten is movably carried by the base for movement between an upper,inoperative position for loading and un loading and a lower operativeposition in which it is spaced a predetermined distance above the lowerplaten. In operative position, its lower face extends horizontally andis also preferably flat. Both platens are substantially circular inplanform and have flange-like margins adapted to be engaged by means forpreventing vertical separation.

In order to provide the downwardly opening cavity to receive the formingunit a tension ring is attached to the upper platen. This ring isannular in planform and has a uniform cross section throughout itsperiphery to preclude stress concentrations. The connection isaccomplished by the use of a support ring which is Wrapped substantiallyaround the lower part of the margin of the upper platen and fixedlysecured thereto. The lower edge of the support ring is formed with aninwardly extending flange which underlies the outer edge of the tensionring and supports it just in contact with the lower face of the platen.Thus the tension ring will not separate vertically from the platen butis free to expand and contract radially under forming loads. The supportring is sufliciently larger than the tension ring to provide adequateclearance for expansion of the latter.

The cavity formed by the joining of the tension ring to the upper platenhouses the hydraulically expansible bag and the forming pad in normalfashion, and the upper face of the lower platen provides the support forone or more workpieces and form means or blocks. When the upper platenis lowered to. operative position the tension ring is brought intocontact with the lower platen to constitute a side wall and define withthe two platens a forming chamber. A pair of clamps are movably mountedon the 'base and each consists of an arcuate member of channel shapedcross section with upright webs and inwardly directed flanges. Theflanges are spaced just far enough apart to encompass the flange-likemargins of the platens, and when the clamps are moved to operativeposition their flanges overlie and underlie these margins to take thevertical loads and prevent vertical separation of the platens during theforming operation.

A hydraulic system for the press includes a fluid reservoir, a pump, andconduit means leading to the upper platen to force fluid into the bagand expand it. When the bag expands, it forces the forming pad downtoward the lower platen and into contact with the workpiece to force itto conform to the surface of the form means or block. Sufficient fluidis introduced to fill all of the voids in the chamber, and the hydraulicpressure is substantially uniform against all of the interior surfaces.The pressure against the upper and lower platens is entirely verticaland is resisted by the clamps. The radial pressure is absorbed entirelyby the tension ring which expands slightly and converts the force intohoop tension. Since the tension ring is not locked to either platen, itsexpansion movement is independent, and no corner bending forces can bedeveloped. Stress concentrations are eliminated and fatigue failure isavoided.

Various other advantages and features of novelty will become apparent asthe description proceeds in conjunction with the accompanying drawings,in which:

FIG. 1 is a perspective view of a press embodying the invention;

FIG. 2 is an elevational view, partly in section, of the press of thisinvention;

FIG. 3 is a sectional view in elevation to an enlarged scale, showingcertain details of the invention;

FIG. 4 is a view similar to FIG. 3 showing a modified form of theinvention; and

FIGS. 5 and 6 are diagrammatic illustrations of the different stresseffects in the known construction and the new construction.

The presently preferred form of press incorporating the features of theinvention is illustrated in simplified form in FIG. 1, in which anelongate structural steel base 10 is provided with a hollow lateralextension 12. Fixed'ly mounted in a central position on the base is alower platen 14 having a supporting body portion 16 and a head portion18. The latter is circular in planform and overhangs the body portion topresent a flange-like margin 20. The face 22 extends horizontally and ispreferably flat. Upper, or head, platen 24, shown in more detail in FIG.2, is also circular in planform and is movably mounted on the base formovement from the inoperative loading and unloading position of FIG. 1to the operative forming position of FIG. 2. Extension 12 carries pivotbrackets 26 to which are pivotally connected the rearward extremities 28of arm 30, the latter being raised and lowered as desired by hydraulicjacks 32. Platen 24 is provided with cars 34 which are pivotallyconnected by pins 36 to the forward free end of arm 30.

The enlarged portion 38 of extension 12 contains a fluid reservoir andpump means, not shown, to provide hydraulic power for the variousfunctions of the press. Fluid for the forming operation is supplied toplaten 24 through conduit 40, swivel joint 42, conduit 44, header 46,and conduits 48. Swivel joint 42 is aligned with the pivotal connectionof arm 30.

When platen 24 is lowered to the operative forming position of FIG. 2,the two platens are locked against vertical separation by a pair ofclamp members 50. Each of these members is arcuate in planfor-m andchannel shaped in cross section, having upright webs 52 and upper andlower inwardly extending flanges 54 and 56 which engage above and belowthe margins of the platens. The upper surface of base 10 is providedwith guide tracks 58 extending longitudinally, and the clamp members areprovided with track engaging means 60 to guide the movement of the clampmembers rectilinearly on the tracks toward and away from the platens.The movement is provided by hydraulic servo motors 62 carried onbrackets 64 secured to the ends of base 10. The servo motors aresupplied with pressure fluid through conduits 66 and the inner ends oftheir piston rods 63 are connectcd to the outer walls of the clampmembers.

Turning now to FIG. 2 where the press is shown in closed position readyfor a forming operation, it will be seen that the upper platen 24 isvery similar to the lower platen. it has an upper body portion '79 and alower head portion 72 which is larger than portion 79 to form aflange-like margin 74. A tension ring 76 is connected to platen 24 in adepending position to form with face 82 a cavity 78 for the reception ofthe forming unit, and the two platens and the tension ring cooperate todefine a closed forming chamber 80.

The manner in which tension ring 76 is connected to platen 24 is shownin enlarged detail in FIG. 3. The lower portion of the flange-likemargin 74 is slightly reduced in diameter at 84 and is further providedwith an annular groove 86. Support ring 88 has a channel shaped crosssection with an upright web 90, an upper flange 92 extending inwardly,and a lower flange 94 extending inwardly. Ring 88 is wrapped aroundmargin 74 with its flange 92 seated in groove 86 and its free ends injuxtaposition hut not joined. A series of peripherally spaced machinescrews 96 retain the ring against displacement, and the engagement offlange 92 in groove 86 transmits the gravity load of ring 76.

Flange 94 underlies the outer margin of ring 76, supporting it in aposition just in contact with face 82 of the platen 24 so that there isno significant clearance, but the ring is free to move radiallyindependently of the platen. The inner diameter of support ring 88 issufficiently larger than the outer diameter of tension ring 76 toprovide adequate clearance for the maximum expansion of the latter underany design working load. Since ring 76 works under tension it isimportant to avoid notch effects, so flange 94 extends below itsrectangular cross section. To close the chamber tightly for the formingoperation ring 76 must also contact margin 20 of the lower platen 14 intight relation. Therefore a shallow undercut 98 is formed in the edge ofmargin 20 to provide adequate clearance for flange 94. This ispermissible because this is not a high stress area in margin 20.

The diameter of margin 74 is reduced at 84 to the extent necessary toaccommodate the thickness of support ring 88. Therefore the webs 52 ofclamp members 50 can be moved into close contact with the maximumdiameters of margins 74 and 20. The spacing of flanges 54 and 56corresponds, to tolerance limits, with the total height of the assemblyof the tension ring and the two platen margins so that when the clampmembers are closed as in FIG. 2 there is maximum area of contact fortransfer of stress.

The difference in the effect of forming loads on conventionalconstruction and on the new construction is diagrammatically illustratedin FIGS. 5 and 6. Both cases represent the head portion of a presshaving a cavity in which hydraulic pressure is applied uniformly to allof the interior walls. In the conventional construction shown in FIG. 5a cavity 109 is formed by a flat head member 102 and a depending andsurrounding side wall 104 integral with the head member 102. The marginof the head member is restrained against vertical movement by a ring orseries of blocks 166. It will be seen that the resultant V of thevertical fluid pressure applies a beam load to head member 102 which iscounteracted by the downward force of member 106. The resultant H of thehorizontal or radial fluid pressure tends to expand side wall 104 but,since the wall is integral with member 102, it is not free to expandradially and force H applies a bending moment about the corner zonewhere member 102 is restrained by ring 106. The result is that at theinside corner indicated at 108 tension forces of varying magnitude arepresent in all directions as indicated by the arrows. These tensions areapplied and relieved with each operation of the press, and the repeatedconcentration of tension stresses in this notch type inner corner leadsto early fatigue failure resulting in cracks such as indicated at 119,which force scrapping of the part.

In the construction of FIG. 6, the head member 112 is a simple platformand the side wall 114 is a tension ring freely slidable with respect tomember 112 and cooperating with it to define the cavity 116. Ring 118restrains member 112 against vertical movement when the resultantvertical force V is applied to it, and the member reacts in simple beambending. The resultant H of the horizontal or radial fluid pressure isabsorbed by wall or ring 114 in hoop tension. Since the ring isindependent of member 112 it expands in response to the load and nocorner bending forces are built up. There is no notch effect to producestress concentrations and the simple stresses in the two parts, althoughhigh, are easy to calculate and to resist. Thus the source of cornerfatigue cracks is eliminated.

Returning to FIG. 2, it will be seen that cavity 78 contains anexpansible pressure-fluid receiving container or bag 120 made uppreferably of a circular flat plate 122 of metal and an elastomericdiaphragm 124, the latter extending around the margin of plate 122 andhaving a bead 126 seated in a groove 128 in the upper surface of theplate. The plate lies adjacent face 82 of platen 24 and is held inposition by conduits 13% which are continuations of conduits 48 passingthrough the platen and the plate and secured to the latter. Thecontainer 1220 extends substantially entirely across the diameter of thecavity and, when pressure fluid is pumped in through conduits 130, thediaphragm stretches and expands downwardly with its marginal portionscontacting the inner wall of tension ring 76.

A resilient, elastomeric, pressure-applying pad 132 extends across thecavity 78 immediately beneath container 120. It is preferably made ofrubber or neoprene of about 70 Shore hardness and is held in place asshown by virture of being slightly larger in diameter than the cavityand being forced into place to bind on the cavity wall. If desired, itmay additionally be secured to the central portion of the diaphragm by anon-drying tacky or adhesive material. The pad is forced down by theexpanding container or bag 120 toward upper face 22 of lower platen 18until it contacts workpiece 134 mounted on form means or block 136seated on face 122. Continued expansion of the container forces themargins of the workpiece down around the side walls of the form blockuntil it conforms to the surface of the form block.

Although the upper and lower platen and the tension ring are heldtightly together during the forming operation, the small tolerancevariations and the distortions resulting from the high loads usuallyproduce small finite clearances suflicient to permit extrusion of rubberand similar materials under the very high fluid pressures. Therefore aseal ring 138 is provided at the junction of platen 24 and tension ring76. This metallic ring, preferably aluminum, is generally triangular incross section with a concave inclined face to match the curvature of thediaphragm. Since it is elastic and of small cross sectional area thefluid pressure forces it tightly against both the platen and the tensionring. It is force fitted into the tension ring and follows it inexpansion and contraction.

Another seal ring 140 is force fitted into the lower end of tension ring76 to seal the junction of the tension ring with lower platen 18 andfunctions in the same way as ring 138. It has a simple triangular crosssection. In addition to the sealing function it also assists inretaining pad 132 in place.

It will be noted that piston rod 68 of servo motor 62 is attached to theouter wall of clamp member and that there are no holes or otherdiscontinuities in the inner wall of Web 52 to cause stressconcentrations. The tension loads in this wall are very high and stressconcentrations here would have serious consequences.

In some fields of work it is necessary to use rather high form blocksrequiring more vertical space than is shown at 80 in FIG. 2. The addedspace has been obtained in some installations by providing a bowl shapeddepression in the upper face of the lower platen, but such constructionresults in the same dimculty with corner bending stress concentrationsas in upper platens. The present invention provides a similar solutionfor this problem. A modified press is illustrated in FIG. 4 in which theupper platen 24, tension ring 76, support ring 88, expansible container120, pad 132, and seal rings 138 and are the same as those of FIGS. 2and 3.

The bowl-shaped depression is achieved by providing a modified lowerplaten 142 which has an upper flat face 144 and an upstanding marginalflange 146. A second tension ring 148 is mounted on face 144 and is ofslightly smaller diameter than flange 146 to allow freedom of movementfor radial expansion. It is also of smaller diameter than the flange 94of the support ring to provide similar radial clearance. The innerdiameter of ring 148 is made smaller than that of tension ring 76 sothat it will underlie the seal ring 140 and it has a sloping inner wall150 to receive the pad. The lower end of wall 150 is made cylindricaland a third seal ring 152 is force fitted therein to serve in the sameway as the other seal rings. The combination produces a deep cavity 154to receive the high form block 156 and its workpiece 158.

The flange 146 provides adequate clearance for any radial exansion ofring 148 under forming loads and at the same time prevents accidentalIateraLdispIacement of the ring. It is not necessary to secure the ringagainst vertical separation from its platen because it is not raised,lowered, or tilted during any phase of normal operation of the press.

While only single form blocks and workpieces are shown, it is obviousthat the press in either form is capable of forming several workpiecesat one time, the capacity being limited only by the relative sizes ofthe workpieces and the forming cavity. It will be seen that the novelpress herein disclosed will accomplish all of the functions ofconventional diaphragm type presses while eliminating the major factorwhich contributes to their short lives.

It will be apparent to those skilled in the art that various changes andmodifications may be made in the construction and arrangement of partsas disclosed without departing from the spirit of the invention, and itis intended that all such changes and modifications shall be embracedwithin the scope of the following claims.

I claim:

1. Apparatus for conforming sheet metal to predetermined shapes on formmeans, comprising: an elongate base; a lower platen fixedly mountedgenerally centrally on said base and having a horizontally extendingupper face to receive a sheet metal workpiece on form means; said lowerplaten being generally circular in planform with a flange-like margin;an upper platen having a lower face and movably connected to said basefor movement between an upper inoperative position and a lower operativeposition in which its lower face extends horizontally in spaced relationdirectly above said lower platen; said upper platen being generallycircular in planform with a flange-like margin or substantially the samediameter as that of the lower platen; an annular tension ring ofsubstantially the same diameter as the platens secured to the upperplaten to move vertically therewith but with freedom of radial movementwith respect thereto; said tension ring cooperating with said upperplaten to define a cavity and being in juxtaposition with the lowerplaten in operative position to convert the cavity to an enclosedforming chamber; an expansible, pressure-fluid receiving container insaid cavity adjacent to the upper platen; a

resilient, pressure-applying pad in said cavity adjacent to thecontainer and movable thereby toward said lower platen to apply formingpressure to a workpiece; a pair of clamp members arcuate in planform andchannel shaped in cross section having upright webs and inwardlyextending upper and lower flanges; guide tracks extending longitudinallyof said base; track engaging means on each clamp member to providelongitudinal guided movement; and servo motor means at the ends of saidbase connected to said clamp members to move them horizontally betweeninoperative position free of said platens and operative position inwhich the upper and lower flanges respectively overlie and underlie themargins of said upper and lower platens to prevent vertical separationduring a forming operation.

2. Apparatus for conforming sheet metal to predetermined shapes on formmeans, comprising: a lower platen having a horizontally extending upperface to receive a sheet metal workpiece on form means; an upper platenhaving a horizontally extending lower face and located directly abovesaid lower platen and vertically spaced therefrom; endless tension ringmeans having an inner surface and located between said platens and injuxtaposition therewith in operative position to define an enclosedpressure chamber; the inner surface of said tension ring meanscomprising substantially the entire verical wall extent of said chamber;an expansible pressurefluid receiving container adjacent to said upperplaten and extending across said chamber; a resilient, pressure-applyingpad in said chamber beneath and adjacent to said container and movablethereby toward said lower platen to apply forming pressure to aworkpiece; and said tension ring means being independently radiallyexpansible and contractible with respect to said platens in response tovariations in forming pressure in said chamber; and said apparatusincluding means to prevent vertical separation of said ring means andplatens during a forming operation.

3. Apparatus as claimed in claim 2; said tension ring means beingannular in planform to define an annular pressure chamber and being ofconstant cross section at all points around the periphery to achieveuniform hoop tension stress per unit area throughout the peripheryduring a forming operation.

4. Apparatus as claimed in claim 2; said tension ring means comprising apair of tension rings; one ring being associated with the upper platento provide a cavity to receive the pressure-fluid receiving containerand the pressure-applying pad; and the other ring being associated withthe lower platen to provide a cavity to receive form means and aworkpiece.

5. Apparatus as claimed in claim 2; one of said platens being movablewith respect to the other to expose the upper face of the lower platenfor loading and unloading of form means and workpieces.

6. Apparatus as claimed in claim 2; said means to prevent verticalseparation comprising a plurality of clamp members each having agenerally channel shaped cross section with upright webs andhorizontally extending upper and lower flanges; said clamp members beingmovable between inoperative position free of said platens and operativeposition in which the upper and lower flanges respectively overlie andunderlie upper and lower marginal portions of said platens.

7. Apparatus as claimed in claim 2; and in addition, a seal ring in saidchamber at the juncture of the tension ring means and the upper platento prevent extrusion of the container therebetween; and a seal ring insaid chamher at the juncture of the tension ring means and the lowerplaten to prevent extrusion of the pad therebetween.

8. Apparatus as claimed in claim 7; said seal rings being metallic andbeing press-fitted into said tension ring means to remain in firmengagement therewith during expansion and contraction under workingloads.

9. Apparatus as claimed in claim 2; said tension ring means comprisingan annular tension ring having a uniform generally rectangular crosssection; and means secured to said upper platen and loosely engagingsaid ring to hold it vertically substantially in contact with said upperplaten while permitting relative radial movement to accommodateperipheral and radial expansion of said ring under working loads.

10. Apparatus as claimed in claim 9; said upper platen being circular inplanform and of substantially the same diameter as said tension ring;and said means to hold said tension ring comprising a support ringsubstantially surrounding said upper platen and fixedly secured theretoand having an inwardly directed flange at its lower end to underlie saidtension ring and support it adjacent to said upper platen.

11. Apparatus as claimed in claim 10; the upper marginal corner of thelower platen being undercut to provide clearance for the flange of saidsupport ring to insure direct contact of said tension ring with thelower platen and minimize extrusion clearances.

12. A press head for use in metal forming presses of the type whichutilize fluid expansible containers for developing forming pressure,comprising: a head platen having a generally horizontal extending lowerface and having a generally circular margin; a member depending from themargin of said platen to define therewith a downwardly opening cavity toreceive a fluid expansible container; said member comprising an annulartension ring of substantially the same diameter as the platen and havinga uniform cross section throughout its peripheral extent to eliminatestress concentrations; said ring being in direct contact with the lowerface of said platen but separate therefrom to provide for radialexpansion of said ring independently of said platen and total separationof radial pressure loads absorbed by said ring from vertical pressureloads absorbed by said platen; and means fixedly secured to said platenand slidably connected to said ring to maintain the latter in directcontact with the platen while permitting limited radial movement of saidring.

13. A press head as claimed in claim 12; said means to maintain the ringin contact with the platen comprising a support ring substantiallysurrounding said platen and fixedly secured thereto; said support ringhaving an inwardly extending flange at its lower edge underlying saidtension ring; said support ring being sufficiently larger than saidtension ring to provide clearance for the maximum radial expansion ofsaid tension ring under working loads.

14. A press head as claimed in claim 13; said platen having an annulargroove in its marginal side wall; and said support ring having aninwardly extending flange at its upper margin seated in said groove totransfer the gravity load of said tension ring to said platen.

15. Apparatus for conforming sheet metal to predetermined shape on formmeans, comprising: a lower platen having a generally horizontallyextending upper face and having a horizontal planar margin; an upperplaten having a generally horizontally extending lower face and having ahorizontal planar margin; said upper platen being located directly abovesaid lower platen and vertically spaced therefrom with said margins invertical registry; endless tension ring means located between saidplatens and having upper and lower surfaces in juxtaposition with themargins of said upper and lower platens when in operative position todefine an enclosed pressure chamber; said tension ring means having aninner surface which comprises substantially the entire vertical wallextent of said chamber; yieldable wall means extending horizontallyacross said chamber and dividing it into first and second compartments;said first compartment being adapted to receive a sheet metal workpieceon form means; and means to supply pressure fluid to said secondcompartment to move said yieldable wall means toward said firstcompartment to apply forming pressure to a workpiece therein; saidtension ring means being independently radially expansible andcontractible with respect to said platens in response to variations informing pressure in said chamber; and said apparatus including means toprevent vertical separation of said ring means and platens during aforming operation.

16. A press head for use in metal forming presses, comprising a headplaten having a generally horizontally extending lower face and agenerally circular margin; an annular tension ring carried by saidplaten and extending below the lower face thereof to define with saidface a downwardly opening cavity constituting a cylindrical pressurechamber; said tension ring having an upper marginal portion injuxtaposition to the margin of said platen but separate therefrom toprovide for radial expansion and contraction of said ring independentlyof said platen and total separation of radial pressure loads absorbed bysaid ring from vertical pressure loads absorbed by said platen; andmeans secured to said platen and said ring to maintain them in constantvertical relation While permitting limited radial movement of said ringwith respect to said platen.

References Cited FOREIGN PATENTS 7/1961 Canada.

CHARLES W. LANHAM, Primary Examiner.

RICHARD J. HERBST, Examiner.

A. RUDERMAN, Assistant Examiner.

